fix: close race conditions in script builder session and slug creation

- script_builder endpoint: pg_advisory_xact_lock on user_id before
  session count check, preventing concurrent creates from both passing
  the MAX_SESSIONS_PER_USER guard
- script_builder_service send_message: pg_advisory_xact_lock on session_id
  before message count check, preventing concurrent sends from both
  passing the MAX_MESSAGES_PER_SESSION guard
- script_builder_service save_to_library: replace check-then-insert slug
  logic with IntegrityError retry loop (3 attempts with fresh UUID suffix);
  add unique constraint on script_templates.slug (migration 070)
- ScriptBuilderPage: add creatingSessionRef to serialize concurrent
  handleSend calls that would otherwise both call createSession() while
  session is still null

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/script_template.py

@@ -48,7 +48,7 @@ class ScriptTemplate(Base):
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
     )
     name: Mapped[str] = mapped_column(String(200), nullable=False)
-    slug: Mapped[str] = mapped_column(String(200), nullable=False, index=True)
+    slug: Mapped[str] = mapped_column(String(200), nullable=False, unique=True, index=True)
     description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
     use_case: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
     script_body: Mapped[str] = mapped_column(Text, nullable=False)